An optical wedge is a kind of prism that is shaped like a wedge and has a face extending from a narrow end to a thick end and transverse to a base, a window at the thick end orthogonal to the base and transverse to the face, and two sides orthogonal to the base, window and face. The face and window are the primary optical input and output, depending on whether the wedge is being used for projection or imaging. The window may be tilted with respect to or at an angle orthogonal to the bisector angle of the wedge. The wedge may be a single pass or double-pass wedge system, ‘stingray’ wedge system or ‘blade’ wedge system.
A wedge serves as an angle-space to position-space converter, because angular field-of-view (FOV) at the wedge thick-end window corresponds to position space across the wedge face. For imaging applications, rays are guided from the wedge face, bouncing off each subsequent opposing wedge surface (which adds twice the wedge angle for each bounce) until their angle-of-incidence is lower than that allowed by total internal reflectance (TIR). Light then escapes on the window at a point corresponding to the angle of input at the face. The escaping light is then collected by a camera lens that directs light to a camera sensor.
Such systems rely on the effect of total internal reflection (TIR) to guide light. However, the transition between TIR and non-TIR is not a pure transition, and a partial amount of light is accepted by the wedge, according to the Fresnel reflectivity curve, near the TIR transition angle. For imaging applications, the light from an object is significantly coupled into the wedge and the first bounce beyond TIR, and then is coupled into the wedge opposing face. However, at the same time, some portion of light from the same object location, having a slightly different angle, is allowed to bounce off an additional face before being coupled into the wedge. This portion of light causes a ghost image of the original object to appear to exist at a different angle within the camera field of view. There can be multiple ghost bounces. The first ghost can exhibit an intensity that typically has on order of 15% to 25% of the original image, while subsequent ghosts are substantially and progressively reduced. However, if the original image is saturated, even two or three ghost images can be visible in the resulting image captured using the wedge. One problem is that both the ghost direction and the ghost pitch changes with respect to position, primarily due to the change in thickness versus position in a wedge.
For the case of a double pass imaging wedge, such as a wedge ‘blade’, the thick end Fresnelated reflector may be curved so as to substantially collimate the guided light along wedge after being reflected by the thick-end reflector. Thus, ghosting may be primarily oriented along the wedge.
For the case of a ‘stingray’ imaging wedge, at least one face of the wedge is non-flat, having a profile which may typically be swept in a radial fashion at a point of origin, typically near the end window where a camera would be placed. Thus, ghosting may have a direction substantially radial from the origin, which would vary with respect to a two-dimensional position on the wedge face. While the ghosting direction may be substantially radial from the origin, the resulting distorted image from a vision system viewing into wedge window may also show such substantial radial direction nature of the ghosting. However, there may be additional slight curvature placed on the image due to lens distortion, such as barrel or pincushion distortion.
In addition to ghost images, other artifacts can arise in an image captured using wedge optics, due to quantifiable physical effects of the optical system. Such effects can arise from, for example, a diffuse layer, diffraction due to a turning film layer, and/or diffraction due to an LCD/display cell aperture array. Such artifacts include, but are not limited to, blurring.
In some wedge imaging systems, attempts have been made to reduce ghosting and blurring by using anti-reflective materials or a cladding layer or by an engineered surface having stand-off structures to form a cladding out of air. All of these solutions increase manufacturing cost and complexity. Further, anti-reflection coatings may be optimized for a given wavelength at a given high angle of incidence; however, optimizing for a range of wavelengths as well as a range of angles typically involves tradeoffs on at least a portion of the band of wavelengths or range of angles, which can limit maximum performance of the layer.